The present invention relates generally to ion traps, and more particularly to a waveform generation method for a three dimensional quadrupole ion trap mass spectrometer simultaneously using both amplitude modulation and frequency modulation.
Three dimensional quadrupole ion traps typically consist of a hyperbolic ring electrode capped by two opposite hyperbolic electrode endcaps. The ring and the endcaps define an interior space wherein ions are trapped by oscillating electric fields generated between the ring and the endcaps. Most quadrupole ion traps apply radio frequency voltage to the endcaps and ring electrodes. By modulating the frequency of voltage applied to the endcaps, ions of a particular mass-to-charge ratio m/z can be excited and/or ejected from the trap.
Quadrupole ion trap mass spectrometers conventionally feature two active modes: an isolation mode and an excitation mode. In the isolation mode, ions of undesirable m/z are ejected from the ion trap by applying a large endcap voltage at a range of frequencies resonant with m/z outside of a selected range. In the excitation mode, remaining ions are excited with lower endcap voltages at a range of frequencies resonant with m/z in the selected range.
Both excitation and isolation modes utilize frequency bandpasses to selectively excite or eject particular ion masses. A variety of methods for creating bandpass waveforms are known in the art. Some conventional waveform generation methods for quadrupole ion traps use a comb of summed, equally-spaced fixed frequencies distributed across an excitation or isolation band. Other conventional methods use Fourier transforms or frequency modulation. Some prior art methods can produce nonuniform or imprecise bandpasses with large discontinuities in amplitude, scattering significant amounts of power outside the intended bandpass region.